1. Field of the Invention
The invention relates to a method of steering road vehicles having front-wheel and rear-wheel steering.
2. Description of the Prior Art
In the steering of road vehicles, such as automobiles, trucks, buses and the like, at present it is required of the driver that he understands the dynamic behavior of the vehicle under different operating conditions, such as load, road condition, travelling speed, etc., and adapts his driving thereto. Admittedly, the assessment of limit situations, such as icy road or narrow curves, will always remain the responsibility of the driver; however, before such limits are reached there are considerable differences between an experienced driver and a beginner and between various vehicles.
The difficulties are known which can arise on occurrence of unexpected yaw movements of the vehicle, for example on a wet road, sudden avoidance maneuver, driving into a side wind at the edge of a wood or behind a bridge, etc. By incorrect steering reactions in such situations the yaw movements can then even be further intensified.
Many traffic accidents are due to driver's errors and are caused in particular by incorrect assessment of the vehicle dynamics at different speeds, different road surfaces and different loads. Existing vehicle steering systems provide the driver with only very inadequate assistance, if at all.
Hitherto, steering a constantly maintained steering angle is converted to a stationary circular path with constant radius and associated yaw rate. However, what the stationary value is and how the transient operation proceeds depends on the changing operating conditions.
In vehicles offered on the market today the steering is frequently power assisted; however, there is always a mechanical connection from the steering wheel to the front-wheels, i.e. the steering wheel position commands the steering angle of the front wheels with a small delay.
In experimental vehicles and also in the literature, for example in "Mechatronics for cars: integrating machines and electronics to prevent skidding on icy roads", by Y. Hossam El-Deen and Ali Seireg in Computers Mechanical Engineering, 1987, p. 10 to 22, the possibility of a so called "drive-by-wire" system has also already been investigated; here, the steering wheel angle is measured, for example by a potentiometer tap, which is then a command variable of an electrical control circuit comprising sensors and of a control law which is implemented in a microcomputer and by means of which an electrical or hydraulic motor is driven for steering the front wheels. The measured steering angle is compared with a reference input to the so called command variable.
Although the introduction of the drive-by-wire system on a large scale is inhibited today by safety doubts similar to those expressed twenty years ago about the corresponding fly-by-wire system in aircraft, it is in particular the positive experience gathered in air travel with fly-by-wire systems which speak for the introduction of the drive-by-wire system. Secondly drive by wire is being introduced for rear wheel steering. Also future possible developments, for example automatic tracking as being investigated in Europe in the so called Prometheus program as well as in the USA and Japan require a drive-by wire system.
An additional rear-wheel steering was first introduced by Japanese manufacturers. For example, Honda employs a mechanical connection by which the rear wheels are steered only in dependence upon the steering wheel angle, in the same sense for smaller angles and in opposite senses for larger angles. Mazda for example have developed a drive-by-wire system for the rear wheels, the ratio of the steering angles at the front and rear being varied in dependence upon the vehicle velocity. In their drive-by-wire system for the rear-wheel steering Nissan and Mitsubishi additionally utilize a pressure measurement from the front-wheel power-assisted steering.
In a Ford test vehicle the yaw rate r is measured with a gyroscope and introduced into the rear-wheel steering system. The yaw rate r is regarded as additional controlled variable, i.e. from the steering wheel angle and the travelling velocity a fictional yaw rate r.sub.ref is calculated and the difference (r.sub.ref -r) is passed onto the rear-wheel steering via a controller specifically designed for the vehicle (see B. Los A. Matheis, J. E. Nametz and R. E. Smith, Construction and development of a vehicle with microprocessor-controlled all-wheel steering, VDI Report no. 650, Dusseldorf, 1987, p. 239-257).
The idea of using a fictituous yaw rate reference signal r.sub.ref is found in various other publications. However, very inconsistent particulars are given on the calculation or determination of the value r.sub.ref which is determined in each case empirically or also theoretically from the stationary circular travel.